Bolt clamping force sensor and clamping force validation method

ABSTRACT

A method to accurately determine and assure proper clamping force or bolt tension in fastening systems comprises statistically sampling washer or flanged bolt lots for clamping force versus deflection relationships, encoding the lots and subsequently installing the fasteners to the prescribed deflections of the lots. A validator and validation method is disclosed that provides the statistical sampling and a corresponding sensor device is disclosed for measuring the deflection as the fastener is installed. Thus, the deflection of the washer or bolt flange as the fastener is installed determines the clamping force applied by the fastener and assures that the proper clamping force has been obtained. The new method and sensor are applicable to special flanged bolt and standard bolt-Belleville washer combinations. No modification of the bolts or washers is required.

BACKGROUND OF THE INVENTION

The field of the invention pertains to fasteners and means to ascertainthe holding or clamping force of an installed fastener. In particular,the invention pertains to threaded fasteners for retaining two or moreparts together, however, the invention is not limited to threadedfasteners but may also apply to non-threaded fasteners where sufficientclamping force must also be assured.

In the typical installation the torque required to turn a threadedfastener is used to indirectly indicate the clamping force or assurethat sufficient clamping force has been applied. Unfortunately, thetorque measured is affected by a variety of parameters only partiallycontrollable. Inaccurate, mismatched, or cross threaded engagement willresult in a false torque indication, as will contaminants such as oils,greases or burrs and abrasive particles. Thus, a torque that appears tomeet specifications may in actuality be insufficient or well beyond thetorque required for the proper clamping force.

In order to avoid the difficulties with torque measurements, more directmeans of determining the clamping force have been developed. Inparticular, modifications to the bolt or the washer under the bolt headhave been developed and met limited success. U.S. Pat. No. 4,773,272discloses a washer with a tab disposed in a radial groove. Thedeflection of the tab as the bolt is tightened provides a measure of thebolt clamping force.

Japanese Patent Document 54-71676 discloses a hollow bolt with a centralpin in the bolt. Above the head of the bolt are a pair of measuringrings separated by a gap. As the bolt is tightened the gap closesthereby closing an electric circuit when the proper bolt tension isreached.

U.S. Pat. No. 4,294,122 and U.S. Pat. No. 3,969,960 disclose ultrasonicmeans to measure bolt tension. Both disclose contact with the head ofthe bolt, however, the former requires use of a transducer in the bolthead and the latter requires bolts in which the stretch constant isknown. U.S. Pat. No. 4,823,606 discloses a bolt having a diaphragmtransducer and strain gages disposed within the bolt to sense loading onthe bolt.

A special washer that plastically deforms or crushes under the bolt headat the desired clamping force is disclosed in U.S. Pat. No. 4,333,220with means to sense the plastic deformation and automatically halt thetightening means. U.S. Pat. No. 4,359,906 discloses a device fortightening a screw into bone and determining the applied torque toprevent overtightening of the screw and damage to the bone.

Most of the devices above require substantial modification of the boltor washer which adds considerable expense and limits the applicationsfor the bolts and washers. In addition a complicated measuring device isrequired in most of the examples. Thus, means are needed to directlymeasure bolt tension or clamping force in standard bolt and washerinstallations without significant additional expense.

SUMMARY OF THE INVENTION

It is an object of the invention to provide accurate means to indicatethe bolt tension or clamping force in bolted fastening systems usingstandard bolts, washers, screws and nuts as required. The invention isalso applicable to special design flange head bolts or nuts where thewasher or flange deflects upon installation. Although disclosed below interms of a standard bolt and conical washer combination, the inventionis applicable to other fasteners where the deflection can be measured inthe same manner.

Bolts, washers and bolt-washer combinations in controlled manufacturedproduction lots will have minimal performance variations, one part toanother, within each lot, however, the variation from lot to lot may besubstantially greater. For example, the force versus deflectionrelationship to compress a conical washer varies little within aproduction lot of washers. Thus, in any well manufactured lot thewashers or fasteners will deflect essentially identically under the sameinstallation circumstances. There may, however, be dramatic differencesfrom one manufactured lot to another, caused by variations indimensions, materials, heat treatments and coatings, for example.

Disclosed below is a validation method to statistically sample eachmanufactured lot to confirm the force versus deflection rate for thelot. A validator is disclosed comprising a calibrated load sensingdevice, a controlled load applicator and a deflection indicator. Theresults of a statistical sample can then be printed on or applied topackaging labels for the lot or incorporated into a bar code for thelot. Known statistical process control (SPC) common to the automobileand appliance manufacturing industries can be applied to the forceversus deflection data from the validator. Capability indexes "C_(p) K",mean values and statistical averages can be calculated for a sample lotwith software such as the "DataMyte", DataMyte Corporation, Minnetonka,MN. In particular, a deflection under a specified load can be computedas a mean or average maximum deflection for the sample set of the washerlot.

The sensor mechanism on the bolt driving unit can then be calibrated tothe lot being utilized on the basis of the statistical validation of thelot. The sensor mechanism comprises a contact means adjacent the boltdriver that senses or measures deflection of the conical washer underthe bolt head or deflection of a bolt flange. With the known lot forceversus deflection relationship the actual clamping force can bemonitored as the bolt is tightened. As disclosed in one embodiment thesensor mechanism can accommodate misalignment of the bolt driver andsensor.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C illustrate the principle upon which the boltclamping force sensor is based;

FIG. 2 shows in schematic cross-section the force sensor mechanism inunloaded position;

FIG. 3 shows in schematic cross-section the force sensor mechanism upontightening of a bolt;

FIG. 4 shows in schematic cross-section an alternate form of the forcesensor mechanisim in unloaded position;

FIG. 5 illustrates in perspective view the exterior of a bolt drive andforce sensor mechanism;

FIGS. 6A and 6B illustrate schematically the flexibility of thealternate form of the bolt drive and force sensor mechanism of FIGS. 4and 5;

FIG. 7 is a schematic partial cross-section of a validator forstatistical performance determination of washer lots; and

FIG. 8 is a system flow chart for the manufacture of the fasteners anduse of the fasteners with the bolt clamping force sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Illustrated in FIGS. 1A, 1B and 1C is a very common fastener used in theautomotive and appliance industries. The fastener comprises a bolt 10having a rolled thread 12 and a conical washer 14 thereon. As shown thefastener connects two plates 16 and 18, plate 18 having a threaded holeengaging the rolled thread 12. In FIG. 1A the bolt 10 is driven to theextent where the conical washer 14 positively engages the plate 16 andunderside of the bolt 10 head but no significant load has been appliedto the conical washer as indicated by the base line 0. As the bolt isdriven further in FIG. 1B the conical washer 14 deflects as indicated bythe 0.010 inches deflection 20 from the base line 0. As will be furtherdescribed below this deflection represents a specific clamping force onthe bolt 10 and plates 16 and 18 as determined from statistical analysisof the washer lot from which the washer 14 was obtained. Doubling thewasher 14 deflection 20 0.020 inches as shown in FIG. 1C effectivelydoubles the clamping force on the bolt 10 and the plate 16 and 18. Thus,over the range of deflection of the conical washer 14, the clampingforce holding the plates 16 and 18 together can be directly determinedby measuring the deflection and applying a force per unit deflection todetermine the force.

In FIG. 2 a mechanism to sense the deflection of a conical washer 14beneath a bolt 10 head is illustrated as part of a drive tool generallydenoted by 22. The drive tool comprises a housing 24 and drive shaft 26.To the lower end of the drive shaft 26 a drive socket 28 is attached bya ball detent 30. The socket 28 in turn is engaged with the head of thebolt 10. The sensing mechanism comprises a sleeve 32 in engagement withthe conical washer 14 as shown and at the upper end in engagement with athrust bearing 34. Atop the thrust bearing 34 is a transducer 36 and acoil spring 38 and atop the coil spring 38 is a snap ring 40 inengagement with the inside of the housing 24. Thus, the spring 38 urgesthe sleeve down into contact with the washer 14. To limit the downwardmovement of the sleeve 32 a second snap ring 42 engages the sleeve and ashoulder on the socket 28.

In FIG. 2 the transducer 36 may be selected from any number of suitabledevices such as a linear variable differential transformer, an air gageor an optical gage. The device need only accurately and repeatably senserelative movement of the sleeve 32 to the drive tool 22. As shown inFIG. 3 the sleeve 32 moves upward 44 relative to the drive tool 22 asthe bolt 10 is driven 46 and the conical washer 14 flattened. Over therange from contact as shown in FIG. 2 until complete flattening isimminent, the deflection of the conical washer 14 bears a linearrelationship with the tension or clamping force of the bolt 10. Thislinear relationship is sensed by the relative motion in the sensor 36and with suitable minimal processing transformed into a clamping force.Thus, by selecting a suitable washer (washer lot) the required clampingforce specified can fall within the linear deflection range of thewasher. The drive tool 22 can be set to automatically stop and retractas the specified deflection (clamping force) is reached or the sensormay be used to merely indicate deflection. Although the sleeve 32 isshown resting upon the washer 14, in some situations the washer or bolthead flange is not of sufficient diameter to provide for contact by asleeve outside the socket 28, thus, in the alternative the sleeve 32 canbe set or calibrated to rest on the plate 16.

Typically, the user establishes the "0" point for the sensor transducer36 by installing a fastener from the lot in a set up gage or actualjoint and tightening until clearances are taken up between the fastener,washer and mating surface (plate 16). With socket 28 and sleeve 32 incontact, the tranducer 36 signal can be adjusted to zero clamping forceor deflection. Any further movement of the sleeve 32 relative to thesocket 28 and tool 22 indicates compression of the washer 14 (deflectionand clamping force).

In FIG. 4 a modified drive tool generally denoted by 48 includes anexternal housing 50 and internal drive shaft 52. To the lower end of thedrive shaft 52 a socket 54 is attached by a universal joint 56. Thesocket engages a bolt 10 head as above and a sleeve 58 engages a conicalwasher 14. A light duty coil spring 60 surrounds the universal joint 56to urge the universal joint 56 and socket 54 into alignment with thedrive shaft 52 when the socket is not engaged with a bolt. Downwardtravel of the sleeve 58 is limited by the snap ring 62 and upwardrelative movement of the sleeve 58 causes compression of the fluidfilled bellows 64 and an upward force on the spring disc 66. The springdisc 66 is fixed to the housing 50 by a shoulder and snap ring 68. Thedrive shaft 52 engages the housing 50 with a bearing 70 and the socket54 engages the sleeve 58 in a sliding fit at 72. Thus, the drive shaft52 and socket 54 rotate relative to the sleeve 58. A boot 74 serves toprevent debris from entering the sensor and universal joint area withinthe drive tool 48. In case of angular misalignment between the driveshaft 52 and the bolt 10, the universal joint 56 and fluid filledbellows accommodate the misalignment and the relative upward movement ofthe sleeve 58 is evenly transmitted to the spring disc 66. Strain gageson the spring disc 66 sense the deflection with an electrical outputtransmitted through the wires 76.

Illustrated in FIGS. 5, 6A and 6B is the exterior of the alternate formof drive tool 48 disclosed in FIG. 4. The sleeve 58 surrounds the socket54 and the boot 74 engages the sleeve 58 and the housing 50. Thus, thedrive tool 48 may operate in proper alignment with the axis of the bolt10 as shown in FIG. 6A or in the event of some angular misalignment theinternal universal joint permits the tool 48 to drive the bolt 10 asshown in FIG. 6B. In both instances the sleeve 58 contacts the conicalwasher 14 about the bolt head and the internal fluid filled bellowspermits the sensing device (spring disc with strain gages) to accuratelymeasure the deflection of the conical washer 14.

As briefly noted above in mass production manufacturing where tens andpossibly hundreds of thousands of bolt-washer fasteners are used a meansof statistically validating the clamping force versus deflection of theconical washers is necessary and the new method of validating is asfollows. Illustrated in FIG. 7 is a simple hand operated validatorgenerally indicated by 78. The validator 78 comprises a receiver 80 andscrew support 82 fixed to a base 84 and a slide 86 moveable back andforth 88 by a manually operated screw 90. The receiver 80 includes acalibrated load cell 92 having a clearance hole 94 for a bolt 96. Theload cell 92 is electrically connected to a force monitor 98. Contactingthe load cell 92 is a sample conical washer 100.

The slide 86 includes a plunger 102 which directly contacts both thehead of the bolt 96 and the screw 90. Thus, operation of the screw 90directly forces the washer 100 against the load cell 92 and the force onthe washer is directly measured. A sensor sleeve 104 contacts the washer100 at the same diameter as the drive tool sleeves 32 and 58 in FIGS. 2and 4 respectively. The sensor sleeve 104 engages an indicator arm 106in turn engaged by a spring 108 within the slide 86. Thus, the spring108 urges the sensor sleeve 104 tightly against the washer 100. Theindicator arm 106 also engages a digital indicator 110 mounted on theslide 86 as shown. Thus, as the screw 90 drives the bolt 96 head andwasher 100 against the load cell 92, the sensor sleeve 104 and arm 106move relative to the slide 86 and bolt 96 as indicated at 112 and asmeasured by the indicator 110.

As a part of the validation method the force (98) versus deflection(110) data from a sample plurality of washers from a specific productionlot is transmitted to a statistical analyzer 114 and, on the basis ofthe sampling, a statistical average output output force versusdeflection provided to a printer 116. The printer 116 may provide a barcode (pounds per inches deflection, for example) that can be affixed tothe cartons or pallets of fasteners comprising the production lot.

For example, standard manufacturing techniques are used for fastenerssuch as disclosed herein. Lot control is currently used on boltsincorporating a conical washer in their design. Lots for such washer andbolt combinations change with material changes in thickness, chemistry,hardness, geometric changes from tooling, etc. Such changes in lots areconsidered normal operating procedures in industry and as such the newvalidation procedure does not add additional burden to the manufactureof the washers or bolt-washer combinations.

After manufacture, a statistical sample of each lot would be removedbefore packaging (such as 30 pieces), and tested with the validatorabove. The bar code in pounds per inches of deflection, for example, isaffixed to the lot. Upon receipt of each lot of fasteners the user cansimply calibrate the drive tool control system to the lot from the barcode affixed to the lot.

FIG. 8 illustrates the system flow or method for determining andassuring that the proper clamping load is applied to each fastener as itis assembled into a final product. Lot control 118 is applied to themanufacture of washer lots 120 and complete bolt-washer fastenersprocessed at 122 with heading, threading, heat treat, finishes, etc.indicated at 124. The finished bolt-washer combinations at 126 arestatistically sampled at 128 with the validator 78 method above and barcoded or other labeling 130 applied to the packaging at 132.

At this point the lot pass to the user at the application point 134 andthe label or bar code scanned 136 to calibrate the drive tool controlsystem to the lot. At the same time a sample bolt-washer is removed 138from the lot and used to zero set 140 the drive tool (see FIGS. 1 and4). The entire lot can thus be utilized 142 with assurance that boltclamping forces will meet specifications.

Returning to FIGS. 2 and 3 this version of the drive tool 22 is directedto robotic and automatic fastener installation systems where thefasteners are automatically fed and high tolerance items in fixtures arejoined together. Examples are cylinder heads and oil pans joined toengine blocks, transmission components joined together and bearing capsinstalled. These joints often require gaskets and typically multipledrive tools tighten a plurality of fasteners simultaneously. With thenew sensor each drive tool acts independently to sense the requireddeflection and clamping force, therefore, stresses inducing deflectionsin the mating parts can be accommodated and the prescribed clampingforce applied at each fastener to provide an even clamping force to thegasketed joint.

The drive tool version of FIGS. 4, 5 and 6 allows for the misalignmentof the drive tool to the fastener. This drive tool is directed to handoperated uses and automatic systems for attaching joints with irregularsurfaces.

The drive tool may be used as an indicator or gage to inspect previouslyinstalled fasteners. With the "0" point previously known inspection ofcritical safety joints on bridges or boiler piping flanges, for example,can be checked.

For best statistical control of clamping force in finished joints in thecase of conical washer type fasteners, specific washer design andmanufacturing criteria should be met.

1. The washer must be hardened or otherwise constructed of a materialthat reacts in the manner of a linear spring.

2. The formulas for Belleville springs apply and specifically theunderneath crown height to thickness ratio should be held between 0.3and 0.6 to one.

3. The washer should never completely flattern before proof load on thebolt is reached, unless specifically designed to do so.

4. Maximum washer deflection should be held to 90% or less of theoriginal crown height, and be at a minimum 0.4 mm (0.015 inches). Thiswill generally allow the sensor sleeve mechanism adequate movement toaccurately detect deflection without sophisticated and expensive linearmovement sensors in the drive tool.

Fortunately actual manufacturing practices can easily meet thesecriteria for conical washers commonly in use. Only good lot controldiscipline is required.

I claim:
 1. A fastener clamping force validation method comprising thesteps of:selecting a sample of deflectable fastener components from amanufactured lot of the components, determining the clamping forceversus deflection relationship for each selected sample component,applying a statistical analysis technique to the set of clamping forceversus deflection relationships for the sample to arrive at a clampingforce versus deflection relationship that statistically represents theclamping force versus deflection relationship for the entiremanufactured lot of the components, calibrating a drive tool sensingmeans to the statistical clamping force versus deflection relationshipof the manufactured lot of the components, and installing the fastenerswith the drive tool to meet the deflection specified in the relationshipthereby assuring that the clamping force has been obtained on astatistical basis.
 2. The method of claim 1 wherein a random sample ofcomponents is selected from the manufactured lot.
 3. The method of claim1 wherein the clamping force versus deflection relationship is linear.4. The method of claim 1 wherein the statistical clamping force versusdeflection relationship is printed in the form of a bar code and appliedto the manufactured lot.
 5. The method of claim 4 wherein the drive toolsensing means is calibrated by reading the bar code applied to themanufactured lot.
 6. The method of claim 1 wherein the statisticalclamping force versus deflection relationship is an average of thedeflections of the sample for a specific clamping force applied.